Apex domains cannot have CNAME records.
Question and response
Question
dig 51.121.27.in-addr.arpa. NS
Answer
51.121.27.in-addr.arpa. 5 NS ns010-ipf8u757epgpe2tf.i.d-53.net.
51.121.27.in-addr.arpa. 5 NS ns010-ipf8u757epgpe2tf.i.d-53.info.
51.121.27.in-addr.arpa. 5 NS ns010-ipf8u757epgpe2tf.i.d-53.jp.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0